An electrophotographic light-sensitive material may have various structures depending upon the characteristics required or an electrophotographic process to be employed.
An electrophotographic system in which the light-sensitive material comprises a support having thereon at least one photoconductive layer and, if necessary, an insulating layer on the surface thereof is widely employed. The electrophotographic light-sensitive material comprising a support and at least one photoconductive layer formed thereon is used for the image formation by an ordinary electrophotographic process including electrostatic charging, imagewise exposure, development, and, if desired, transfer.
Furthermore, a process using an electrophotographic light-sensitive material as an offset master plate precursor for direct plate making is widely practiced. Particularly, a printing system using a direct electrophotographic printing plate has recently become important for providing high quality prints of from several hundreds to several thousands.
Binders which are used for forming the photoconductive layer of an electrophotographic light-sensitive material are required to be excellent in the film-forming properties by themselves and the capability of dispersing photoconductive powder therein. Also, the photoconductive layer formed using the binder is required to have satisfactory adhesion to a base material or support. Further, the photoconductive layer formed by using the binder is required to have various excellent electrostatic characteristics such as high charging capacity, small dark decay, large light decay, and less fatigue due to prior light-exposure and also have an excellent image forming properties, and the photoconductive layer stably maintains these electrostatic characteristics regardless of change of humidity at the time of image formation.
Further, extensive investigations have been made on lithographic printing plate precursors using an electrophotographic light-sensitive material, and for such a purpose, binder resins for a photoconductive layer which satisfy both the electrostatic characteristics as an electrophotographic light-sensitive material and printing properties as a printing plate precursor are required.
However, conventional binder resins used for electrophotographic light-sensitive materials have various problems particularly in electrostatic characteristics such as a charging property, dark charge retention and photosensitivity, and smoothness of the photoconductive layer.
In order to overcome these problems, JP-A-63-217354 and JP-A-1-70761 (the term "JP-A" as used herein means an "unexamined Japanese patent application") disclose improvements in the smoothness of the photoconductive layer and electrostatic characteristics by using, as a binder resin, a resin having a weight average molecular weight of from 1.times.10.sup.3 to 1.times.10.sup.4 and containing at random an acidic group in a side chain of the polymer or a resin having a weight average molecular weight of from 1.times.10.sup.3 to 5.times.10.sup.5 and having an acidic group bonded at only one terminal of the polymer main chain thereby obtaining an image having no background stains.
Also, JP-A-1-100554 and JP-A-1-214865 disclose a technique using, as a binder resin, a resin containing an acidic group in a side chain of the copolymer or at the terminal of the polymer main chain, and containing a polymerizable component having a heat- and/or photo-curable functional group; JP-A-1-102573 and JP-A-2-874 disclose a technique using a resin containing an acidic group in a side chain of the copolymer or at the terminal of the polymer main chain, and a crosslinking agent in combination; JP-A-64-564, JP-A-63-220149, JP-A-63-220148, JP-A-1-280761, JP-A-1-116643 and JP-A-1-169455 disclose a technique using a resin having a low molecular weight (a weight average molecular weight of from 1.times.10.sup.3 to 1.times.10.sup.4) and a resin having a high molecular weight (a weight average molecular weight of 1.times.10.sup.4 or more) in combination; and JP-A-1-211766 and JP-A-2-34859 disclose a technique using the above low molecular weight resin and a heat- and/or photo-curable resin in combination. These references disclose that, according to the proposed techniques, the film strength of the photoconductive layer can be increased sufficiently and also the mechanical strength of the light-sensitive material can be increased without adversely affecting the above-described electrostatic characteristics achieved by using a resin containing an acidic group in a side chain or at the terminal of the polymer main chain.
On the other hand, in order to evaluate electrostatic characteristics of electrophotographic light-sensitive materials, values of E.sub.1/2 and E.sub.1/10 which are obtained based on exposure amounts corresponding to times required for decay the surface potential to 1/2 and 1/10, respectively are conventionally employed. These two values are important factors for evaluating reproducibility of original in practical image formation. More specifically, as the values of E.sub.1/2 and E.sub.1/10 are small and a difference thereof is small, clear duplicated images without blur can be reproduced.
In addition, another point at the image formation is a degree of electrical potential remaining in the exposed area (non-image area) after light exposure. When the degree of remaining electrical potential is high at the image formation, background fog is formed in the non-image area of duplicated images. An electrostatic characteristics mainly corresponding to this subject is a value of E.sub.1/100. The smaller the value, the better the image forming performance.
In particular, in a recent scanning exposure system using a semiconductor laser beam, the value of E.sub.1/100 becomes an important factor in addition to the charging property (V.sub.10), dark decay retention rate (DRR) and E.sub.1/10 conventionally employed, since there is the restriction on the power of laser beam.
In case of using a resin having a low molecular weight and containing an acidic group and a resin having a high molecular weight or a heat- and/or photo-curable resin in combination as above described known techniques, the V.sub.10, DRR and E.sub.1/10 are reached to a substantially satisfactory level. However, it has been found that the value of E.sub.1/100 obtained in the case of changing the environmental conditions or in the case of using a laser beam of low power is not sufficient and background fog occurs in duplicated images.